Thermoplastic substrates such as polycarbonates and poly(methyl methacrylate) are generally characterized by their many advantageous properties which include clarity, high ductility, high heat deflection temperature, as well as dimensional stability. Many of these materials are transparent and are conventionally employed as replacements for glass in many commercial applications. However, such materials easily scratch and abrade, resulting in possible decrease in transparency. They often display low chemical solvent resistance and are susceptible to degradation by ultraviolet light. This results in unfavorable characteristics including yellowing and erosion of the substrate surface.
As a result, various methods to improve the weatherability and abrasion resistance of thermoplastic substrates have been developed. Such methods conventionally employ treating the thermoplastic substrate surface with a coating material, whereby the coating material typically contains ultraviolet light absorbing agents such as benzophenone and benzotriazole derivatives that decrease yellowing due to weathering conditions. The coating material may also contain a silicon compound for improved abrasion resistance. Coatings with increased abrasion resistance include so-called "silicone hardcoats", which are thermally cured, and silicon compound-containing compositions which are capable of being cured by radiation, such as ultraviolet (UV) radiation.
Ultraviolet light (i.e. radiation curable) curable abrasion resistant coating compositions are dispersions of hard colloidal silica filler in acrylic monomers which can be cured using a free radical type of photo initiator. The ultraviolet radiation-cured coating compositions are advantageous because of their short curing times. These coatings provide increased weatherability, or abrasion resistance or a combination of weatherability and abrasion resistance to the underlying thermoplastic substrate.
Many ultraviolet radiation-curable abrasion-resistant coatings are known in the art. U.S. Pat. No. 4,455,205 discloses compositions comprising a silyl acrylate, aqueous colloidal silica, a photoinitiator and optionally a polyfunctional acrylate. Other materials which may be present include absorbers of ultraviolet radiation, employed as stabilizers, and hindered amines.
Other types of ultraviolet radiation-curable coating compositions are disclosed in U.S. Pat. No. 4,486,504, which contain colloidal silica, a silyl acrylate, a polyfunctional acrylate and a photoinitiator. U.S. Pat. No. 4,491,508 contains colloidal silica, a silyl acrylate, a polyfunctional acrylate and a photoinitiator. Both of these patents also disclose the optional presence of ultraviolet stabilizers or compounds convertible thereto. U.S. Pat. No. 4,863,802 discloses coating compositions of essentially the same types further containing an ultraviolet radiation-absorbing amount of a dimeric benzotriazole compound as a stabilizer. Similar compositions employing acylphosphorus compounds such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide are disclosed in U.S. Pat. No. 5,162,390, which also discloses the use of benzotriazoles and benzophenones as ultraviolet stabilizers.
Recently, it is becoming more and more common for thermoplastic substrates such as polycarbonate to be employed outdoors. It is thus important to impart weatherability properties on the ultraviolet radiation-curable coatings. Weather resistant coating systems can be prepared by incorporating ultraviolet absorbers, such as benzotriazoles and benzophenones, and hindered amine light stabilizers. However, prolonged exposure to sunlight, moisture and thermal cycling conditions can cause yellowing, delamination and formation of microcracks, decreasing transparency. These conditions are often noted in varying degrees in the compositions disclosed in the aforementioned patents, even when one of the properties mentioned is improved weatherability.
The instant invention, therefore, is directed to novel and improved ultraviolet light curable coatings, herein referred to as UV cured coatings, which are capable of absorbing ultraviolet light to increase the weathering characteristics of the coating. The instant invention further provides coating compositions that greatly improve weatherability of the coating and coated article by decreasing yellowing, delamination, and the formation of microcracks, as compared to presently known coatings being used for their weathering characteristics.